icmp: add extensions for interface and next-hop identification

This change implements ICMP extensions for interface and next-hop
identification which are used for route trace applications as described
in RFC 5837.

Change-Id: I2435109b5e766e743b894b0280a537324975489d
Reviewed-on: https://go-review.googlesource.com/3112
Reviewed-by: Ian Lance Taylor <iant@golang.org>
6 files changed